The long-term goal is to understand the processing of human speech sounds in the auditory nervous system. Studies of the processing of both speech and nonspeech stimuli in the chinchilla inferior colliculus (IC) are proposed. Responses of single neurons will be measured to determine the distributed response to consonant-vowel syllables that differ in voice onset time (VOT). Both humans and chinchillas hear syllables differing in VOT as belonging to qualitatively different categories separated by a perceptual boundary. Both species also exhibit increased acuity for VOT syllables adjacent to the category boundary, and decreased acuity for syllables within each category. The experiments will examine neural mechanisms that may contribute to the formation of VOT categories; this aspect of speech sound processing is important but is poorly understood. The results of these experiments could lead to better speech processors for use with hearing aids, or with cochlear or brainstem implants intended to provide speech information to profoundly-deaf patients. Specific Aim I will test three hypotheses about the representation of VOT in the IC: that the temporal pattern of the population response to a given syllable is related to characteristic frequency; second, that the temporal pattern covaries with other response properties such as the shape of the histogram elicited by a pure tone; and third, that the information that can be observed in the neural representation varies nonmonotonically across the continuum of VOT syllables in agreement with the pattern of psychophysical acuity. Measurements will be made with a limited stimulus set, from as many single neurons as possible, and from all parts of the IC. This approach is analogous to the "population study" approach that has been essential for understanding the coding of speech in the auditory periphery. Specific Aim 2 will test the hypothesis that the representation of VOT in the IC is nonmonotonic despite changes in interaural time or intensity. Specific Aim 3 will test the hypothesis that the region of the VOT continuum that is represented with the greatest precision shifts, in agreement with shifts in the location of the psychophysical boundary for VOT. Specific Aim 4 will test the hypothesis that responses to VOT syllables can be predicted from pure tone response properties.